Our "Sunday spotlight" shining on a Hollywood a-lister who is putting his star power behind a humble cause. Matt Damon is throwing himself and glamorous friends into improving water and sanitation in the developing world. ABC's see seal Cecilia Vega with the inspiring story. Reporter: From a janitor turned Harvard math genius in good will hunting. Well, I got her number, how do you like them apples? Reporter: A trained cia assassin in the Bourne trilogy -- to this? What invention has saved the most lives? A toilet. Let's cut the crap. Reporter: Matt Damon, toilet advocate. The Oscar winner, a-lister, the sexiest man alive, is taking the plunge in his most unique role yet. Learning about this, water just underpinned everything. It's just so huge. There's a real opportunity to save a lot of people. Reporter: In this country, you don't think twice about the luxury of walking into a bathroom and using a toilet. What's at stake? People's lives. Every 20 seconds a child dies because they lack access to clean water and sanitation. Every 20 seconds, three kids every minute somewhere on planet Earth. Not here. Our kids aren't going to die from diarrhea that's just an inconvenience in the west. But it is a stark, terrifying reality to billions of people on the planet. That's why we have a sense of urgency. The deaths are unnecessary. Reporter: He staged a protest. Until everybody has access to clean water and sanitation, I will not go to the bathroom. Reporter: And enlisted very famous friends in a little bathroom humor. An online spoof with a serious message gone viral. It's easy for me, I have never gone to the bathroom in my entire life. And remember, if you don't use the toilet, you're a celebrity. Reporter: Four years ago, Damon co-founded water.org with environmental engineer Gary white. They have traveled the world from Haiti to Africa to India bringing access to safe drinking water and sanitation. Building wells and toilets, and even offering microloans to some of the poorest populations. Instead of drilling a well and giving it for free. What we're looking at is how do we help them get access to a small loan so they can get a water connection from the local utility and become a customer. Reporter: You're empowering. : Exactly. If you give a loan and essentially buy their teem back, right, they can connect to the water source. They can have a tap in their house and they can get those hours back to work. Reporter: Those who benefit most? Women and girls. And for this husband and father of four daughters, that is a big deal. They are the ones doing the water collections. The girls are the ones who are leaving school and to the going to school because their job is to fetch water for the family. So if you can get them access to the water, the outlook changes. Suddenly she can get an education and have hope for a bright future rather than a future around scavenging for water to survive to the next day. Here's the water connection. Wow. Reporter: And his hope for the future? That this role as a smart philanthropist will be the biggest impact yet. For "This week," I'm Cecilia Vega, ABC news, Los Angeles. Our thanks, and some welcome news. No deaths of service members in Afghanistan this week. That's all for us today. Thanks for sharing part of your Sunday with us. Check out "World news" with David Muir tonight. Have a great day."world news with David

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